Mixing up Orders Wasn't an Accident

So last night at work was a doozy. Usually I pride myself on being able to keep track of all the drinks and never mix anyone's order, but tonight, something weird happened. I mixed up two orders for customers sitting next to each other. One guy wanted his drink extra strong with no soda water added, while the other asked for their usual light pour. When I brought them their drinks, they both looked at me like I had a third eye in my forehead. I'm usually so careful because it's not just about getting fired; mixing up orders can really mess with someone's night and make them question my skills as a bartender. I know I shouldn't have let my mind wander during such an important task, but tonight, the customer who asked for his drink light was one of those people you just find yourself thinking is extra annoying. Maybe that's why I screwed up; maybe it was the sheer boredom of repeating the same routine over and over again. Whatever it was, it felt like a slap in the face. Now, here I am wondering if it's just an off day or if I'm losing my touch.